Looking back at the current INPUT_LISTENER_CB_DEFINE api naming, it
feels like it's a bit overloaded. Rename it to a simpler
INPUT_CALLBACK_DEFINE.
Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>
Remove all init functions that do nothing, and provide a `NULL` to
*DEVICE*DEFINE* macros.
Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>